feat: add remove entry from map

[Aursen]

No description provided.

[uint]

Hi!

Adding remove is a welcome addition, but it'd have to be implemented on the ItemAccess type instead.

https://docs.rs/storey/latest/storey/containers/struct.ItemAccess.html

Map/MapAccess itself doesn't always "hold a value" under a given key - it might just have another nested Map. The remove operation is a little confusing then:

let map = Map::<String, Map<String, Item<u32>>>::new(0);

map.access(&mut storage).entry_mut("foo").entry_mut("bar").set(&42).unwrap();

map.access(&mut storage).entry_remove("foo"); // what are we removing? there's nothing under the "foo" key in the storage backend

[uint]

See previous comment

[uint]

LGTM!

[uint]

Suggested change

	///
	///
	/// item.access(&mut storage).set(&42).unwrap();

Not a must, but this probably makes a better doc test here.